Tunable Josephson Generator for Qubit Excitation and Two‐Tone Spectroscopy

ABSTRACT

We present a tunable Josephson generator that enables direct qubit excitation and two‐tone spectroscopy while reducing cryogenic wiring complexity and thermal load. The device consists of a current‐biased Josephson junction integrated with a superconducting spiral resonator, forming a microwave source that operates at cryogenic temperatures. Unlike conventional qubit drive setups that require coaxial microwave lines from room temperature to the base stage, our system requires only a single direct current (DC) bias line from room temperature. The generated microwave signal is delivered to the qubit via a short coaxial line between the 4 K and 20 mK stages, significantly reducing wiring complexity, thermal load in the cryostat and leveraging unused volume at the 4 K stage. We demonstrate continuous‐wave excitation of a superconducting qubit and perform tunable two‐tone spectroscopy using the Josephson generator as the drive source, resolving qubit spectra with performance comparable to a standard microwave source. Our generator is characterized by low phase noise and can be phase‐locked to an external reference for enhanced stability. Operating at 4 K temperature, this self‐contained source offers a low‐noise, space‐efficient solution for qubit control and spectroscopy, paving the way for scalable cryogenic quantum control architectures.